Integrins are a superfamily of cell surface receptors that mediate cell-cell and cell-matrix adhesion. These proteins are known to provide anchorage as well as signals for cellular growth, migration and differentiation during development and tissue repair. They have been implicated in immune and inflammatory processes.
Integrins are heterodimeric proteins composed of two noncovalently linked polypeptide chains, α and β. The amino terminus of each chain forms a globular head that contributes to interchain linking and to ligand binding. The globular heads are connected to the transmembrane segments by stalks. The cytoplasmic tails are usually less than 50 amino acid residues long. Integrin subfamilies were originally defined on the basis of which β subunit was used to form the heterodimers. The β1-containing integrins are also called VLA molecules, referring to “very late activation” antigens. VLA-1 to VLA-6 refer to β1 subfamily members containing α1 to α6 (i.e., CD49a to CD49f), respectively. For general review, see Cellular and Molecular Immunology, eds. Abul K. Abbas et al., W. B. Saunders Company, Philadelphia, Pa., 2000.
Collagen (both types I and IV) and laminin are known ligands of α1β1 integrin (i.e., VLA-1). VLA-1 has been implicated in cell adhesion and migration on collagen (Keely et al., 1995, J. Cell Sci. 108:595-607; and Gotwals et al., 1996, J. Clin. Invest. 97:2469-2477); in promoting contraction and reorganization of collagen matrices, a critical component of wound healing (Gotwals et al., supra; and Chiro, 1991, Cell 67:403-410); and in regulating the expression of genes involved in extracellular matrix remodeling (Riikonen et al., 1995, J. Biol. Chem. 270:1-5; and Langholz et al., 1995, J. Cell Biol. 131:1903-1915). Thus, improper regulation of VLA-1 may result in certain pathological conditions such as fibrosis.
Moreover, it has been suggested that VLA-1 may play a role in T cell/monocyte-driven diseases. Anti-VLA-1 antibodies block T-cell dependent cytokine expression (Miyake et al., 1993, J. Exp. Med. 177:863-868). Expression of VLA-1 is increased in persistently activated, 2 to 4 week old cultured T cells (Hemler et al., 1985, Eur. J. Immunol. 15:502-508). VLA-1 is also expressed on a high percentage of T cells isolated from the synovium of patients with rheumatoid arthritis (Hemler et al., 1986, J. Clin. Invest. 78:692-702).
Several crystal structures of integrin a subunits have been determined, including the structures of the α2-I domain of α2β1 (PDB accession code 1aox; Emsley et al., 1997, J. Biol. Chem. 272:28512-28517); the α1-I domain of rat α1β1 (PDB accession number 1ck4; Nolte et al., 1999, FEBS Lett. 452:379-85; WO 00/20459); the α1 subunit of human α1β1 (PDB accession code 1qc5; Rich et al., 1999, J. Biol. Chem. 274:24906-24913); the αL-I and αM-I domains; and vWF-A3 (Lee et al., 1995, Cell 80:631-635; Lee et al., 1995, Structure 3:1333-1340; Qu et al., 1995, Proc. Natl. Acad. Sci. USA 92:10277-10281; Qu et al., 1996, Structure 4:931-942). The α2β1 structure revealed a helix (i.e., the C-helix) that created a trench or groove on one face of the protein at the metal-ion binding site (Emsley et al., supra). The crystal structure of the α2-I domain complexed to a short collagen-based triple helical peptide revealed that the collagen-based peptide was bound to that trench, where the α2-I amino acids that made intermolecular or metal contacts included Asp151, Asn154, Tyr157, Gln215, Asp219, Leu220, Thr221, Asp254, Glu256, His258, Tyr285, Leu286, Asn289, Leu291, Asn295, and Lys298 (PDB accession code 1dzi; Emsley et al., 2000, Cell 101:47-56; WO 01/73444). The amino acid numbering immediately above is based on PDB accession code 1dzi and herein referred to as “crystal numbering.” The crystal structures of the rat and human α1-I domains revealed a similar trench.